Archaea are a type of single-cell organism which are so different from other modern life-forms that they have challenged the way scientists classify life.
Until the advent of sophisticated genetic and molecular biology studies allowed scientists to see the major biochemical differences between archaebacteria and “normal” bacteria, both were considered to be part of the same kingdom of single-celled organisms. “Kingdoms,” a way of organizing life forms based on their cell structure, traditionally included Animalia, Planitia, Fungi, Protista (for single-celled eukaryotes), and Monera (which was once considered to hold all forms of prokaryotes).
However, genetic and biochemical studies of bacteria soon showed that one class of prokaryotes was very different from “modern” bacteria, and indeed from all other modern life forms. Eventually named “archaebacteria” from “archae” for “ancient,” these unique cells are thought to be modern descendants of a very ancient lineage of bacteria that evolved around sulfur-rich deep-sea vents.
Sophisticated genetic and biochemical analysis has led to a new “phylogenetic tree of life,” which makes use of the concept of “domains” to describe divisions of life that are bigger and more basic than that of “kingdom.”
The most modern version of this system shows all eukaryotes – animals, plants, fungi, and protists – constituting the domain of “Eukaryota,” while the more common and modern branching of bacteria constitutes “Prokarya,” and archaebacteria constitute their own domain altogether – the domain of “Archaea.”
The discovery of Archaea and its unique differences is exciting for scientists, because it’s believed that archaebacteria’s unique biochemistry might give us insight into the workings of very ancient life. Some scientists propose that the archaebacteria Thermoplasma may in fact be ancestors of the nuclei of our own eukaryotic cells, which are believed to have developed through the process of endosymbiosis.
Another remarkable trait of archaebacteria is their ability to survive in extreme environments, including very salty, very acidic, and very hot surroundings. Archaebacteria have been recorded surviving temperatures as high as 190° Fahrenheit, which is only twenty-two degrees shy of the boiling point of water, and acidities as high as 0.9 pH.
Archaebacteria have even challenged scientist’s ideas about how to define a species, since they practice a lot of horizontal gene transfer – where genes are transferred from one individual to another during their lifetimes – making it difficult to determine how closely different cells are related, or even if archaebacteria cells have the sort of stable combinations of traits that scientists typically use to define a species.
The domain of Archaea include both aerobic and anaerobic species, and can be found living in common environments such as soil as well as in extreme environments.
So what biochemical characteristics make scientists so excited about archaebacteria? Well…
Archaebacteria have a number of characteristics not seen in more “modern” cell types. These include:
1. Unique cell membrane chemistry.
Archaebacteria have cell membranes made of ether-linked phospholipids, while bacteria and eukaryotes both make their cell membranes out of ester-linked phospholipids
Archaebacteria use a sugar that is similar to, but not not the same as, the peptidoglycan sugar used in bacteria cell membranes.
2. Unique gene transcription.
Archaebacteria have a single, round chromosome like bacteria, but their gene transcription is similar to that which occurs in the nuclei of eukaryotic cells.
This leads to the strange situation that most genes involving most life functions, such as production of the cell membrane, are more closely shared by Eukarya and Bacteria – but genes involved in the process of gene transcription are most closely shared by Eukarya and Archaea.
This has led some scientists to propose that eukaryotic cells arose from a fusion of archaebacteria with bacteria, possibly when an archaebacteria began living endosymbiotically inside a bacterial cell.
Other scientists believe that eukaryotes descended directly from archaebacteria, based on the findings of archaebacteria species, Lokiarcheota, which contains some found only in eukaryotes, which in eukaryotes code for genes with uniquely eukaryotic abilities.
It is thought that Lokiarcheota may be a transitional form between Archaea and Eukaryota.
Only archaebacteria are capable of methanogenesis – a form of anaerobic respiration that produces methane.
Archaebacteria who use other forms of cellular respiration also exist, but methane-producing cells are not found in Bacteria or Eukarya.
4. Difference in rRNA
Differences in ribosomal RNA that suggest they diverged from both Bacteria and Eukarya at a point in the distant past
Types of Archaebacteria
There are three main types of archaebacteria. These are classified based on their phylogenetic relationship (how closely related they are to each other), and members of each type tend to have certain characteristics. The major types are:
1. Crenarchaeota –
Crenarchaeota are extremely heat-tolerant. They have special proteins and other biochemistry that can continue to function at temperatures as high as 230° Fahrenheit! Many Chrenarchaeota can also survive in very acidic environments.
Many species of Crenarchaeota have been discovered living in hot springs and around deep sea vents, where water has been superheated by magma beneath the Earth’s surface.
One theory of the origin of life suggests that life may have originally started around deep sea vents, where high temperatures and unusual chemistries could have led to the formation of the first cells.
They are able to survive in very salty habitats. They are also able to produce methane, which no other life form on Earth is able to do!
Euryarchaeota are the only form of life known to be able to perform cellular respiration using carbon as their electron acceptor.
This gives them an important ecological niche because the breakdown of complex carbon compounds into the simple molecule of methane is the final step in the decomposition of most life forms. Without methanogens, the Earth’s carbon cycle would be impaired.
Wherever methane gas is produced by life, Euryarchaeota are responsible.
Methanogen archaebacteria can be found in marshes and wetlands, where they are responsible for “swamp gas” and part of the marsh’s distinctive smell, and in the stomachs of ruminants such as cows, where they break down sugars found in grass that are undigestible to eukaryotes by themselves. Some methanogens live in the human gut and assist us in the same way.
They can also be found in deep sea sediments, where they produce pockets of methane beneath the ocean floor.
They are the least-understood and thought to be the oldest lineage of archaebacteria. This makes them possibly the oldest surviving organisms on Earth!
Korarchaeota can be found in hydrothermal environments much like Crenarchaeota. However, Korarchaeota have many genes found in both Crenarchaeota and Euryarcheaota, and also genes which are different from both groups. To scientists, this suggests that both other types of archaebacteria may have descended from a common ancestor similar to Korarchaeota.
Korarchaeota are rare in nature, perhaps because other, newer forms of life are better adapted to survive in modern environments than they are. Still, Korearchaeota can be found in hot springs, around deep-sea vents.
Examples of Archaebacteria
Lokiarcheota is a hyperthermophile discovered at the deep-sea vent called Loki’s Castle, which some scientists think has unique evolutionary significance.
It has a highly unique genome, consisting of roughly 26% proteins that are known to be found in other archaebacteria, 29% proteins that are known to be found in bacteria, 32% genes that do not correspond to any known protein, and – 3.3% genes that correspond to those only found in eukaryotes.
The eukaryotic genes are particularly exciting for scientists, because they are genes that appear to code for proteins that eukaryotes use to actively control the shape of their cell, including proteins for cytoskeletons, the motor protein actin, and several proteins that in eukaryotes are involved in changing cell membrane shape.
Some of these genes are involved in phagocytosis, which is exciting because the process of phagocytosis could have been used by eukaryotic ancestors to “swallow” other cells – which may have gone on to become endosymbiotes, leading to the endosymbiotic relationships between eukaryotic cells and their mitochondria, chloroplasts, and nuclei.
Lokiarchaeota’s unique genome makes it possibly our closest relative among prokaryotes, and possibly a transitional form in the extremely important jump from prokaryotic to eukaryotic life, which made the evolution of the animal, plant, fungi, and protist kingdoms possible. Scientists think that Lokiarchaeota and ourselves probably shared a common ancestor around 2 billion years ago.
It is unknown whether this means that eukaryotes likely evolved around deep-sea vents, or whether Lokiarchaeota’s relatives may once have been common in other environments before they were outcompeted and driven to extinction by their more advanced descendants, the eukaryotes.
Methanobrevibacter smithii is a methane-producing archaebacteria that lives in the human gut. This member of Euryarchaeota helps us to break down complex plant sugars and extract extra energy from the food we eat.
The microorganisms in our guts – including members of Euryarchaeota – also have a complex relationship with our health. While some studies show that many people with obesity and colon cancer have above-average levels of Euryarchaeota in their guts, Euryarchaeota also help people who don’t have enough food to produce more energy, and some types of these archaebacteria appear to protect against colon cancer.